Direct observation of knock-on reaction with umbrella inversion arising from zero-impact-parameter collision at a surface.

In Surface-Aligned-Reactions (SAR), the degrees of freedom of chemical reactions are restricted and therefore the reaction outcome is selected. Using the inherent corrugation of a Cu(110) substrate the adsorbate molecules can be positioned and aligned and the impact parameter, the collision miss-distance, can be chosen. Here, substitution reaction for a zero impact parameter collision gives an outcome which resembles the classic Newton's cradle in which an incident mass 'knocks-on' the same mass in the collision partner, here F + CF → (CF)' + (F)' at a copper surface. The mechanism of knock-on was shown by Scanning Tunnelling Microscopy to involve reversal of the CF umbrella as in Walden inversion, with ejection of (F)' product along the continuation of the F-reagent direction of motion, in collinear reaction.